1. Field of the Invention
This invention related generally to composite materials and more particularly to an alumina-alumina composite.
2. Description of the Prior Art
Alumina (aluminum oxide) Al.sub.2 O.sub.3 is a well known ceramic which is used in applications requiring electrical insulation and/or high temperature operation. When fabricated in a hot-pressed polycrystalline form it has sufficiently good mechanical properties to serve as a structural component for such purposes as laboratory ware, furnace liners and high power vacuum tube insulators. It has also been employed, by using careful design, as a missile antenna window or radome.
Single crystals of sapphire (crystalline alumina) have been used as windows in armored tanks because of the transparency of this crystal to visible portions of the electromagnetic spectrum, as well as its high indentation hardness which provides protection from small arms fire.
Both of these familiar forms of alumina ceramics, polycrystalline and monocrystalline (sapphire) suffer two limitations to their broader use in structural applications where large temperature gradients may be created in parts fabricated from these forms of alumina:
1. They are brittle ceramics, i.e. they fracture at very low strain levels typically less than 0.1%. Thus although they may have high intrinsic tensile strengths they fracture at low strain values and the integrated product of their stress-strain or "fracture toughness" is low. PA1 2. Because of the relatively high coefficient of thermal expansion of alumina, (4.3.times.10.sup.-6 /.degree.F. vs. 0.5.times.10.sup.-6 /.degree.F. for fused silica) high thermal strains are caused in alumina structures in which temperature gradients exist. The low strain capability thus makes them extremely sensitive to thermal shock failure.
Previously, structures have been disclosed which are intended to solve the problem of poor resistance to mechanical and thermal shock of alumina bodies. In U.S. Pat. No. 4,092,194 a yarn of refractory oxide fibers, which may be .alpha.-alumina, is wound on a tubular core. A coating is placed on the wound tube and fired to form a refractory oxide matrix. Different approaches to the matrix fabrication are disclosed. In one approach, a refractory oxide precursor such as alumina hydrate is used to form the coating which is fired. In another approach, an aqueous solution, suspension, dispersion, slurry, emulsion or the like which contains particles of one or more oxides or oxide compounds is used as the coating which is fired. The foregoing patent structure technique is somewhat limited in usefulness however because the fibers extend in only two basic directions.
Three directional (3D) and higher number directional structures which have been fabricated from fiber type materials have also been disclosed and are shown generally in U.S. Pat. No. 4,252,588. The material which has been disclosed in this patent is however a carbon which is not suitable for use in many places where oxide ceramics are used.
In U.S. Pat. No. 4,268,562 a composite is disclosed which uses alumina fibers in a glass matrix. A two directional structure is fabricated by joining layers of coated fibers.
It is therefore an object of this invention to provide an alumina-alumina structure having fiber reinforcement in at least three directions.
It is also an object of this invention to provide a three directional alumina-alumina structure capable of being machined to a desired configuration.
It is a further object of this invention to provide an alumina-alumina structure which is resistant to fracture by thermal expansion or mechanical shock.
It is also an object of this invention to provide an alumina-alumina composite having improved mechanical strain characteristics.
It is a still further object of this invention to provide an alumina-alumina composite having a high dielectric quality (high electric resistance) for use as an electrical insulator, including use as a radar-transmitting window.